Tuberculosis remains a significant public health problem, despite substantial effort to develop new drugs and vaccines in recent years. Host variation in terms of susceptibility to the infection is a major impediment to the development of universally efficient prevention and treatment strategies. This variation is partially explained by the genetic factors. In our studies mouse model of infection with virulent Mycobacterium tuberculosis (MTB) is used to dissect complex genetic control of host resistance to MTB and reveal genes, which strongly influence progression of tuberculosis infection in vivo. Our studies demonstrate that dramatic differences in tuberculosis susceptibility of two immunocompetent mouse strains C3HeB/FeJ and C57BL/6J mice are due to effects of several genetic loci and their epistatic interactions. To date, we have mapped nine tuberculosis resistance loci, dissected the sst1 locus and identified the Ipr1 gene by positional cloning. Synergisitic interactions of the sst1 with a novel locus on mouse chromosome 7 (Chr7) represent a major genetic component in our model, which accounts for about 50% of the variation between the parental strains. The cumulative effect of the two B6-derived resistance loci was remarkably stronger than the sum of their individual effects, providing the first direct experimental evidence of epistatic gene interactions in control of host resistance to MTB. Together the two loci increased the survival of the extremely susceptible C3HeB/FeJ mice from one month to 5 - 6 months post infection, greatly improved control of MTB growth and decreased lung inflammation. In this application we propose genetic and functional characterization of the Chr7 locus to identify causal gene using positional cloning and reveal its specific role in control of host resistance of tuberculosis infection. The candidate region will be reduced to 0.5 - 2 Mb interval using a set of subcongenic strains that we have developed. Haplotype structure, gene expression pattern and sequence analysis of the minimal candidate region will be used to identify top candidate genes. Functional analysis of the candidate gene will be performed using in vitro and in vivo assays to obtain definitive proof for its role in host resistance to MTB and characterize pathways that the candidate gene controls at molecular, cellular and whole organism levels. This will lead to a better understanding of how different pathways interact to produce resistant or susceptible phenotypes in immunocompetent hosts, an exploration of these pathways in humans and potentially provide molecular targets for correction of the susceptible phenotypes. PUBLIC HEALTH RELEVANCE: Tuberculosis remains a significant public health problem. To reduce the disease burden we must improve our understanding of how the pathogen destroys lungs of immunocompetent individuals and reveal environmental and genetic factors responsible for big differences among individuals in resistance to tuberculosis. We use genetic analysis in experimental mouse model of infection to identify genes, which effects strongly influence progression of tuberculosis infection in the lungs and have found two major host resistance loci, which synergistic effect dramatically reduce lung pathology and increase survival of the tuberculosis infected mice. We have previously characterized one of those loci, sst1, and found a candidate gene, which mediates innate immunity to tuberculosis. In this application we propose genetic and functional dissection of the second locus on chromosome 7 to isolate novel host resistance gene and reveal its function in control of tuberculosis infection.